The present invention relates to the mechanical mounting of power electronic devices used in electronic systems.
The power handling capability of electronic components used in power electronic systems is limited by the maximum operating temperature of the device. To make cost effective use of the power devices in a system, an efficient system to remove the heat generated by these components must be used. This usually involves mounting the power devices to a heat sink which removes the heat from the device. In high power electronic systems, there may be many power devices.
During the last decade, the requirement for efficient device cooling has become even more important. It has become feasible to produce reliable, inexpensive devices capable of controlling larger currents. Semiconductor wafer processing improvements have allowed power devices of increasingly large die area to be produced with high yields and hence low cost. Packaging these larger die in inexpensive, industry standard discrete packages such as the TO-220 and the TO-247 (FIG. 2) has contributed to the low cost and popularity of these devices. TO-220 or similar class devices usually have 2 to 5 leads (most commonly 3) for connection on the printed wiring board. The most common versions have exposed metal on the surface to be mounted against the heat sink for good thermal conduction. This exposed metal surface also provides a low resistance and low inductance path to the internal semiconductor die or dice.
Even with the importance of device cooling, it is difficult to obtain low cost, reliable methods of mounting power electronic devices so that they have both an electrical connection to a printed circuit board and a mechanical, thermal and possibly electrical connection to a heat sink.
One attempt to address this need in high power systems has been to increase the integration of the power electronic devices into packages which are more reliably mounted. This approach to the mounting problem has been simply to reduce the number of components to be mounted. However, these higher integration devices are typically 2 to 10 times more expensive than the TO-220 and TO-247 class components on a volt-amp basis. These higher integrations devices are application specific and hence will never have the volume production, and hence lower cost, of the discrete power devices. Furthermore, these higher integration packages are not amenable to automated assembly.
The heat sink is a significant portion of the size, cost and weight of a power electronic system. Designers often use this heat sink as the outer packaging of the system or a power conductor (in which case it is referred to as a bus bar). Therefore, there may be system requirements to have the heat sink electrically isolated from the power devices or the devices may be required to have good electrical conduction to a bus bar.
There are four elements to reducing power device cost in a power electronic system:
a. Use the most inexpensive devices on a volt-amp basis. PA1 b. Use the device at its rated current capacity by ensuring efficient and reliable thermal contact between the device and the heat sink. PA1 c. Use the tab of the device for conduction. This reduces stray inductance so that more efficient, lower voltage devices can be used. PA1 d. Use an inexpensive mounting method which does not rely on tight assembly tolerances. PA1 a. accurate registration in the x, y, z and .theta. directions are required. Inaccurate registration causes (in the best case) reduced thermal contact between the heat sink and the device and (in the worst case) failed devices due to stress cracks. PA1 b. drilling and tapping the heat sink is an expensive operation and inserting the screw is labor intensive PA1 c. the screw must be accurately torqued to prevent bending of the TO-220 device and reducing the area of contact between the device and the heat sink PA1 d. the mounting quality cannot be visually checked. PA1 a. is still not cost effective (spring tooling, bolts, tapped holes and manual assembly) PA1 b. the spring force varies with the cumulative tolerances in the mechanical system PA1 c. Further, the force on the device cannot be easily checked during production. PA1 a. lack of cost effectiveness (spring tooling cost, tool cost, tool wear) PA1 b. the spring force varies with cumulative tolerances PA1 c. the spring compression operation scrapes a hard spring steel against a much softer extrusion or casting which potentially leads to metal scrapings being present during the final assembly process. These filings can move during the product lifetime and potentially cause electrical short circuits, causing the product to fail PA1 d. spring force cannot be easily checked during production PA1 e. one side of the spring is anchored in the printed circuit board. Circuit boards are not structural and the spring forces involved will cause board stresses possibly leading to stress cracks in components or traces. The mounting hole also wears with vibration and this causes the spring pressure to be unreliable. Further, the mounting hole is obtrusive.
Existing power electronic systems use the following methods to mount the power devices to the heat sink, described here as "Techniques A, B and C".
Technique A uses one screw per power device, through the mounting hole in the tab of the TO-220 device into the heat sink. This approach has a number of problems:
Technique B uses springs to press the TO-220 against the heat sink. These springs are compressed using bolts. This is an improvement over the previous system but has these disadvantages:
Technique C (for example, as disclosed in U.S. Pat. No. 5,331,258) uses springs as in technique B but compresses the spring using special production tooling until it locks into place behind a clip. The disadvantages with this system are: